Physical Determinants of Particle Uptake and Transport during Phagocytosis

Abstract

Phagocytosis is a central part of the mammalian immune system and an evolutionary highly conserved fundamental cellular process. A large number of molecules that are involved in phagocytosis are already known. However, a quantitative physical knowledge of this intrinsically mechanical process and therefore a comprehensive understanding is still lacking. Therefore, we investigated physical determinants of particle uptake and transport during phagocytosis. We examined the cellular resolution limit for particle uptake by using holographic optical tweezers in combination with correlative light and electron microscopy to measure the ability of cells to discriminate between two spatially separated objects. These studies provide insights into the spreading of cell signaling and the areas of signal integration during the uptake of particles. Using a combination of traction force microscopy and holographic optical tweezers, we were able to measure cellular forces during phagocytic uptake in a spatially and temporally resolved manner. The subsequent intracellular transport of phagosomes was investigated by live-cell particle tracking. We found that not all phagosomes are transported directly from the cell periphery to the perinuclear region. They rather exhibit more complex transport characteristics, which depend strongly on the size of the phagosomes. This transport behavior might be the foundation for a size-dependent cellular sorting mechanism for organelles. To gain insights into the underlying causes for this process, we quantified how the intracellular transport forces scale with the organelle size by using magnetic tweezers. These measurements provide cues for the numbers of participating motors and their potential cooperative behavior during the transport of different-sized organelles. In summary, we anticipate our results to pave the way for a deeper quantitative understanding of phagocytosis and thus enable the development of new and more comprehensive models for this important cellular process.